It is now well established that there is a hierarchy of organization on chromosomes of higher plants and animals. The basic element of this hierarchy is a repeating subunit particle, the nucleosome. In nature, the nucleosome disks are organized into a 20-30 nm thick fiber which is observed to form the basis of the structure of chromatin and chromosomes. It is proposed to study the structure of the 20-30 nm fiber by electron microscopy. Of particular interest is the architecture of the fiber as to the arrangement of nucleosomes. Various aspects of the fiber structure will be studied as a function of ion type and concentration--factors of known importance to the organization of the fiber. Central to this proposal is the innovative use of dark field scanning transmission electron microscopy (STEM). It is expected that STEM will offer significant improvements over conventional transmission electron microscopy (CTEM) in the direct observation of fiber structure and diversity. The basic idea is to observe detailed architecture in stained and unstained fibers in a manner anticipated to minimize structure alterations. The proposed research will add to our knowledge of chromosome structure. It will improve our ideas about the ways in which chromatin and chromosomes are organized during the various parts of the cell cycle, and it will provide a basis for investigation in the structural elements of genetic control. The development of procedures for STEM will further its use in problems of biological significance, a development which will be important to other problems in structural biophysics.